This project proposes to investigate associations between brain abnormalities detectable with current MRI technology among children under 5 years of age with a first febrile seizure and (1) febrile seizure type, (2) family history of febrile seizures, (3) developmental delays in cognition and behavior, and (4) febrile seizure recurrence. The authors will study these factors in 252 cases with first febrile seizure and 252 febrile children without seizure, frequency matched to cases on age, gender, and month of ascertainment. At baseline, we will compare: (1) the prevalence of brain abnormalities detected on MRI performed within 72 hours of the first febrile seizure by febrile seizure type; (2) the prevalence of febrile seizure in siblings of comparison children compared to cases with and without brain abnormality; and (3) the cognitive and behavioral function of cases with and without brain abnormality and of comparison children. Children will be followed over time for febrile seizure recurrence, and cognitive and behavioral function. This study aims to determine whether: (1) brain abnormalities are more common among children with complex versus simple febrile seizures; (2) brain abnormalities are inversely associated with a family history of febrile seizures in first degree relatives; (3) brain abnormalities are associated with developmental delay; and (4) once the influence of family history of febrile seizures is removed, brain abnormalities are not associated with an increased risk of recurrent febrile seizures. Answers to these questions may improve the clinical management of subgroups of children at risk for long-term disability so that future at-risk children can benefit from early interventions.